The present invention relates to telecommunications and more particularly to synchronizing transceivers in a direct sequence spread spectrum radio communications system.
Modern communication systems, such as cellular and satellite radio systems, employ various modes of operation (analog, digital, and hybrid) and access techniques such as frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and hybrids of these techniques.
Digital cellular communications systems have expanded functionality for optimizing system capacity and supporting hierarchical cell structures, i.e., structures of macrocells, microcells, picocells, etc. The term xe2x80x9cmacrocellxe2x80x9d generally refers to a cell having a size comparable to the sizes of cells in a conventional cellular telephone system (e.g., a radius of at least about 1 kilometer), and the terms xe2x80x9cmicrocellxe2x80x9d and xe2x80x9cpicocellxe2x80x9d generally refer to progressively smaller cells. For example, a microcell might cover a public indoor or outdoor area, e.g., a convention center or a busy street, and a picocell might cover an office corridor or a floor of a high-rise building. From a radio coverage perspective, macrocells, microcells, and picocells may be distinct from one another or may overlap one another to handle different traffic patterns or radio environments.
An typical cellular mobile radiotelephone system includes one or more base stations (BSs) and multiple mobile stations (MSs). A base station generally includes a control and processing unit which is connected to a core network type node like a mobile switching center (MSC) which in turn is connected to the public switched telephone network (PSTN). General aspects of such cellular radiotelephone systems are known in the art. The base station handles a plurality of voice or data channels through a traffic channel transceiver which is controlled by the control and processing unit. Also, each base station includes a control channel transceiver which may be capable of handling more than one control channel also controlled by the control and processing unit. The control channel transceiver broadcasts control information over the control channel of the base station to mobiles tuned (or locked) onto that control channel.
The mobile receives the information broadcast on a control channel at its voice and control channel transceiver. The mobile processing unit evaluates the received control channel information, which includes the characteristics of cells that are candidates for the mobile to lock on to, and determines on which cell the mobile should lock. Advantageously, the received control channel information not only includes absolute information concerning the cell with which it is associated, but also contains relative information concerning other cells proximate to the cell with which the control channel is associated.
In North America, a digital cellular radiotelephone system using TDMA is called the digital advanced mobile phone service (D-AMPS), some of the characteristics of which are specified in the TIA/EIA/IS-136 standard published by the Telecommunications Industry Association and Electronic Industries Association (TIA/EIA). Another digital communication system using direct sequence CDMA PS-CDMA) is specified by the TIA/EIA/IS-95 standard, and a frequency hopping CDMA communication system is specified by the EIA SP 3389 standard (PCS 1900). The PCS 1900 standard is an implementation of the GSM system, which is common outside North America, that has been introduced for personal communication services (PCS) systems.
Several proposals for the next generation of digital cellular communication systems are currently under discussion in various standards setting organizations, which include the International Telecommunications Union (ITU), the European Telecommunications Standards Institute (ETSI), and Japan""s Association of Radio Industries and Businesses (ARIB). An example third operation standard currently being proposed by ETSI is the Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access (UTRA). Besides transmitting voice information, the next generation systems can be expected to carry packet data and to inter-operate with packet data networks that are also usually designed and based on industry-wide data standards such as the open system interface (OSI) model or the transmission control protocol/Internet protocol (TCP/IP) stack. These standards have been developed, whether formally or defacto, for many years, and the applications that use these protocols are readily available.
In most of these digital communication systems, communication channels are implemented by frequency modulating radio carrier signals, which have frequencies near 800 megahertz (MHz), 900 MHz, 1800 MHz, and 1900 MHz. In TDMA systems and even to varying extents in CDMA systems, each radio channel is divided into a series of time slots, each of which contains a block of information from a user. The time slots are grouped into successive frames that each have a predetermined duration, and successive frames may be grouped into a succession of what are usually called superframes. The kind of access technique (e.g., TDMA or CDMA) used by a communication system affects how user information is represented in the slots and frames, but current access techniques all use a slot/frame structure.
Time slots assigned to the same mobile user, which may not be consecutive time slots on the radio carrier, may be considered a logical channel assigned to the mobile user. During each time slot, a predetermined number of digital bits are transmitted according to the particular access technique (e.g., CDMA) used by the system. In addition to logical channels for voice or data traffic, cellular radio communication systems also provide logical channels for control messages, such as paging/access channels for call-setup messages exchanged by base and mobile stations and synchronization channels for broadcast messages used by mobile stations or other remote terminals for synchronizing their transceivers to the frame/slot/bit structures of the base stations. In general, the transmission bit rates of these different channels need not coincide and the lengths of the slots in the different channels need not be uniform. Moreover, third generation cellular communication systems being considered in Europe and Japan are asynchronous, meaning that the structure of one base station is not temporally related to the structure of another base station and that mobile does not know any of the structures in advance.
In such digital communication systems, a receiving terminal must find the timing reference of a transmitting terminal before any information transfer can take place. For a communication system using DS-CDMA, finding the timing reference corresponds to finding the boundaries of downlink (e.g., BS-to-MS) chips, symbols, and frames. These are sometimes called downlink chip-, symbol-, and frame-synchronizations, respectively. In this context, a frame is simply a block of data that can be independently detected and decoded. Frame lengths in today""s systems typically fall in the range of ten milliseconds (ms) to twenty ms. This search for BS timing may be called a xe2x80x9ccell search,xe2x80x9d and it includes identification of BS-specific downlink scrambling codes that are features of current DS-CDMA communication systems.
A mobile or other remote terminal typically receives a signal that is a superposition (sum) of attenuated, faded, and disturbed versions of the signal transmitted by a BS. The slot and frame boundaries in the received signal are unknown to the MS to begin with, as are any BS-specific scrambling codes. The mobile therefore must detect and identify one or more BSs in the noise-like (for DS-CDMA) received signal and to identify the scrambling code used. In order to help synchronize the remote terminal to the BS and identify the BS-specific scrambling code, some communication systems provide that each BS signal includes an unscrambled part, which may be called a synchronization channel (SCH), which the MS can lock onto and perform a xe2x80x9ccell search.xe2x80x9d
Even after downlink synchronization is achieved, synchronization also must be achieved for uplink communications (e.g., MS-to-BS). This is because of the rapidly changing radio interface between the base and mobile stations as well as xe2x80x9croundtripxe2x80x9d propagation delays to and from the base station. When the mobile desires to transmit to the base station, it sends an initial transmission over an uplink channel (e.g., a random access channel (RACH)). Because of the already-achieved downlink synchronization between the base and mobile stations, the uplink transmission is roughly or coarsely synchronized, e.g., typically within one time/access slot. For example, the base station may only allow uplink transmissions from mobiles during eight distinct time/access slots. The mobile knows generally when those time slots begin when it synchronized to the downlink signals being broadcast by the base station. The assumption therefore may be made that the mobile is out of synchronization no more than one such time/access slot. In a direct sequence spread spectrum system, such a time/access slot corresponds to a predetermined number of chips, e.g., 256 chips. In this example, the largest roundtrip propagation delay, i.e., the amount which the mobile is out of synchronization with the base station, is assumed to be 255 chips.
To achieve the synchronization, the one station""s transmission includes some known signal (i.e., known to the other station). In the downlink direction, the known signal is sometimes referred to as a synchronization sequence or code or a primary synchronization sequence or code. In the uplink direction, the known signal may be referred to as a preamble sequence. In the random access, the mobile station transmits the uplink random access burst. The random access burst consists of a preamble (or preamble sequence) part and a message part. The base station RACH receiver correlates the received signal samples with the known preamble sequence. Once the maximum preamble correlation is detected by the base station RACH receiver, the base station is in synchronization and can accurately decode the substantive message from the mobile contained in the message part of RACH burst. Although only one random access synchronization code/preamble is needed for each base station, different synchronization codes/preambles associated with the base station may be used to minimize cross-correlation between uplink transmissions from different mobiles.
In any event, the synchronization code(s)/preamble(s) used for the downlink cell search or for the uplink random access synchronization should have a minimal, out-of-phase, aperiodic autocorrelation. Autocorrelation is the property which describes how well a code or sequence correlates with itself. Minimal, out-of-phase, aperiodic autocorrelation means the autocorrelation values for the non-zero time shifts (i.e., the base and mobile stations are out of synchronization by one or more chips) are low compared to the zero-time shift autocorrelation value (the base and mobile stations are chip synchronized). Non-zero time shift autocorrelation values are called autocorrelation sidelobes. A zero time shift autocorrelation value is called the autocorrelation main lobe.
One possible source for synchronization codes with unit envelope are binary or polyphase Barker codes. The main lobe to maximum sidelobe ratio for Barker codes is equal to L, where L is the code length. Unfortunately, binary Barker codes exist only for lengths 2, 3, 4, 5, 7, 11, and 13, while polyphase Barker codes are currently known for the lengths up to 45. As a result, non-optimum codes have to be used when longer preamble synchronization codes are required. For example, the preamble of random access burst signal proposed for a third generation mobile communication standard (UTRA) has a length of 4096 code chips.
Besides minimal aperiodic autocorrelation and longer code length, it is also important to efficiently generate and correlate the synchronization codes. With respect to correlation at the base station, efficiency is even more important if there are a number of different synchronization preambles/codes which can be randomly used so that the base station random access channel receiver must correlate a received composite signal with all possible synchronization/preamble codes. In this situation, the base station receiver employs a bank of random access preamble correlators, each of which performs a large number of data processing operationsxe2x80x94especially if the code length is long. A similar problem exists with respect to the design and detection of the downlink synchronization codes transmitted by the base station so that the mobile can identify the base station timing and obtain frame, symbol, and chip synchronization (i.e., cell search).
To achieve efficiency, the proposed UTRA random access burst preamble is generated using a serial (or hierarchical) code concatenation procedure which has the benefit of reducing the number of correlation operations. The preamble is generated using a xe2x80x9csignaturexe2x80x9d having 16 complex symbols. There are 16 different signatures obtained from an orthogonal set of binary orthogonal Gold sequences of length 16 by multiplying each binary sequence with the constant complex number C=(1+j)/{square root over (2)}, where j={square root over (xe2x88x921)}. Each signature symbol is spread with a so-called preamble spreading code, chosen to be a 256 chips long orthogonal Gold sequence. The resulting preamble sequence therefore has a length of 4096 chips, i.e., 16xc3x97256=4096. The preamble spreading code is cell specific and is broadcast along with information about allowed signatures in its cell by the base station.
Unfortunately, the aperiodic autocorrelation properties of preambles based on serially concatenated orthogonal Gold sequences are not optimal, i.e., significant autocorrelation sidelobes are generated. Significant autocorrelation sidelobes mean that the receiver may erroneously synchronize to one of these sidelobes, and as a result, the transmitted message is not properly received.
It is therefore desirable to provide one or more synchronization sequences that for longer lengths provide minimal aperiodic autocorrelation properties and which can be generated and correlated efficiently. Such synchronization sequences could then be used advantageously in many types of spread spectrum communications applications such as downlink cell search and uplink random access.
To meet these and other objectives, the present invention provides a correlator that may be included in a first transceiver used to correlate a received signal transmitted by a second transceiver. The correlator includes a matched filter corresponding to a complementary pair of sequences to correlate the received signal with one of the complementary pair of sequences. A detector detects a peak output from the matched filter, and timing circuitry uses the detected peak output to generate a timing estimate for synchronizing transmissions between the first and second transceivers. Each of the complementary sequences has very low, aperiodic autocorrelation sidelobe values for all non-zero delays of that complementary sequence and a maximal autocorrelation main lobe value for a zero delay of that complementary sequence.
In one example application, the first transceiver may be a base station, and the second transceiver may be a mobile station. The one sequence may be used as a preamble portion of a random access burst transmitted by the mobile station to the base station over a random access channel. Alternatively, the first transceiver may be a mobile station, and the second transceiver may be a base station. The one sequence may be used as a synchronization sequence transmitted by the base station over a synchronization or other broadcast type of channel.
A plurality of such matched filters may be employed as a bank of correlators at the first transceiver with each matched filter corresponding to a complementary pair of sequences having minimal aperiodic autocorrelation sidelobe properties. This is particularly advantageous when there is a need for plural sequences from an orthogonal set to be used in the same cell, e.g., plural random access channels used, for example, to increase capacity. In a preferred example embodiment, the complementary pair of sequences is a complementary pair of binary sequences of length L=2N, where N is a positive integer. The complementary pairs of binary sequences are usually called Golay complementary pairs, and sequences forming such pairs are called Golay complementary sequences. The Golay complementary pair of sequences preferably has a relatively long length, e.g., L=4096 chips.
The present invention also provides for a sequence generator that efficiently generates a pair of Golay complementary sequences of length L=2N that requires only N memory elements. A modulo 2N counter having N memory elements cyclically counts from zero to 2Nxe2x88x921. A permutator coupled to receive and permute N parallel outputs from the counter according to a certain permutation of integers ranging from one to N. A first set of N logical AND operators produces N parallel outputs from pairs of adjacent permuted counter outputs. These outputs are then summed modulo 2 in a first summer. A second set of N logical AND operators operates on each permuted counter output and one of a set of N weighting coefficients to produce N parallel outputs. A second summer sums modulo 2 the outputs generated by the second set of N logical AND operators. A third summer sums modulo 2 the outputs of the first and second summers to produce a first binary sequence in the complementary pair of binary sequences. A fourth summer sums modulo 2 the output of the third summer and a most significant output from the permutator to produce a second binary sequence in the complementary pair of binary sequences.
In addition to an efficient synchronization sequence generator, the present invention also provides for an efficient sequence correlator for efficiently correlating a received spread spectrum signal with a pair of binary (Golay) or polyphase complementary sequences of length L=2N. Such a correlator is sometimes referred to as the efficient Golay correlator (EGC). The efficient sequence correlator includes N serially concatenated processing stages, each stage having first and second parallel processing branches. Each stage of the first processing branch includes a delay line coupled to a corresponding adder. Each stage of the second processing branch includes a multiplier coupled to a subtractor. The input signal provided to the first processing branch at the particular processing stage is stored in plural memory elements of the corresponding delay line, while the content of the last memory element of the same delay line is input to the adder (in the first branch) and to the subtractor (in the second branch) of the same stage. The input signal provided to the second processing branch at the same stage is multiplied in the multiplier by a corresponding weighting coefficient. The output of the multiplier is fed to the negative input of the subtractor (in the second branch) and to the adder (in the first branch). The output of the adder is then input to the delay line in the next successive stage. The output of the subtractor is then input to the multiplier in the next successive stage. The input signals for the first and the second processing branches at the first processing stage correspond to the received spread spectrum signal.
The output of the first processing branch at the N-th processing stage produces a cross-correlation of the received spread spectrum signal with a first sequence from a complementary pair of sequences. The output of the second processing branch at the N-th processing stage produces a cross-correlation of the received spread spectrum signal with a second sequence from a complementary pair of sequences. In a preferred example embodiment, the complementary sequences are binary or Golay complementary sequences.
The present invention also provides for a memory efficient Golay correlator with substantially reduced memory for correlating a received signal using one of the sequences from a pair of complementary sequences of L=2N. The one sequence from the complementary pair is represented as a concatenation of two shorter, constituent complementary sequences of length X. Each constituent sequence is repeated L/X times and is modulated by +1 or xe2x88x921 according to a signature sequence of length Y=L/X. A first correlator of length X receives the spread spectrum signal and generates an intermediate pair of correlation values corresponding to the constituent complementary sequences. A first selector alternately supplies one of the intermediate pair of correlation values during each of successive time windows based on a current value of an interleaving sequence of length Y=L/X whose elements belong to the set {0,1}. The order of concatenation of the constituent sequences is determined by the interleaving sequence. A multiplier multiplies the selected intermediate correlation values with the corresponding bits of the signature sequence to generate a multiplied correlation output. The multiplied correlation output is summed with a feedback output generated by a delay line having X memory elements with the resulting sum itself being fed back to the input of the delay line. A final correlation value corresponds to the resulting sum after Y successive time windows have occurred. In a preferred example embodiment, N=12, L=4096, X=256, and Y=16, and the first correlator is an efficient Golay correlator such as the one just described above.
In yet another embodiment, this memory efficient correlator correlates the received spread spectrum signal with plural orthogonal sequences generated using plural signatures. Plural multipliers each multiply the selected intermediate correlation values with an element of a different signature sequence to generate a corresponding multiplied correlation output. Plural summers are provided with each summer corresponding to one of the multipliers and summing the multiplied correlation output with the feedback output of a corresponding delay line. After processing all the elements in the signature sequence, a final correlation value is generated for each of the orthogonal sequences. One example embodiment is disclosed in which sixteen signatures are used to generated sixteen orthogonal Golay sequences. Another example embodiment employs thirty-two signatures to generate thirty-two orthogonal Golay sequences.
As a result of the present invention, a single synchronization sequence or an orthogonal set of synchronization sequences may be provided with excellent aperiodic autocorrelation properties for relatively long sequence lengths. Equally significant, the present invention provides for efficient generation of and correlation with one synchronization sequence or an orthogonal set of such synchronization sequences. These synchronization sequences can be advantageously employed in one example application to uplink synchronization over a random access channel and downlink synchronization over a broadcast synchronization channel.